1. Field of the Invention
The present invention is generally related to processes for sulfur recovery and more specifically to processes for removing sulfur compounds, including hydrogen sulfide and sulfur dioxide, from process streams.
2. Description of the Related Art
Processing of hydrocarbon-containing fuels such as gasoline and diesel fuel results in gases containing sulfur compounds, including hydrogen sulfide (H2S), and hydrocarbon compounds, including ammonia (NH3). Governmental regulations limit plant emissions of sulfur-bearing gases. Refineries commonly include sulfur reduction units to decrease emissions of sulfur compounds.
The use of a Claus reaction to recover sulfur from process off-gases is widely known in the field. Sulfur dioxide (SO2) and hydrogen sulfide react to produce elemental sulfur (S2) and steam (H2O). The reaction formula is:2H2S+SO2→1.5S2+2H2OThe reaction can occur with or without a catalyst. As long as two moles of hydrogen sulfide are available for each mole of sulfur dioxide in appropriate concentrations with appropriate heat and pressure, elemental sulfur and water will result.
A typical prior art arrangement of a Claus process with a SCOT tail gas process is outlined in FIG. 5. The prior art generally teaches high temperature reaction of air, through air line 16, off-gas streams containing hydrogen sulfide, through hydrogen sulfide line 18, and off-gas streams containing ammonia, through ammonia line 12, in a thermal reactor 14 to oxidize a portion of the hydrogen sulfide in the process streams to create sulfur dioxide and to react the sulfur dioxide with hydrogen sulfide into elemental sulfur and water, thus eliminating a substantial portion of the hydrogen sulfide and sulfur dioxide in the streams. The quantity of air to the thermal reactor is controlled to provide a stoichiometric balance of hydrogen sulfide and sulfur dioxide in the process stream. A balanced stoichiometric ratio is difficult to maintain due to variations in composition of process streams.
Commonly, the process stream containing significant hydrogen sulfide is divided prior to introduction to the reactor chamber, with approximately 30-70% of the feed directed to the front end 22 of the thermal reactor chamber 14 through line 18 and the remaining 30-70% directed to the back end 20 of the reactor chamber 14 through line 18a. The mixture of air and off-stream gases is thermally reacted near the front end 22 of the reactor chamber 14 and moves to the back end 20 of the reactor chamber 14 where the second hydrogen sulfide stream is received.
It is desirable to maintain a temperature at or near 1,316° C. (2,400° F.) in the thermal reactor 14 to crack process gas hydrocarbon compounds such as ammonia. Providing a determined quantity of hydrogen sulfide at the front end 22 of the reactor chamber 14 assists in maintaining desired temperature in the thermal reactor 14. An excess of hydrogen sulfide tends to lower the temperature in the reactor chamber 14 due to increased mass flow. In prior art thermal reactors, the temperature at the back end 20 of the reactor 14 may be less than the desired temperature because of added off-gases containing hydrogen sulfide.
After the thermal reactor 14 the resulting gas stream is processed through a series of Claus reactors 46, 60 and 72, condensers 34, 50, 64 and 76 and reheaters 42, 54 and 68. The Claus reactors 46, 60 and 72 typically have aluminum oxide or bauxite catalyst beds. The Claus reaction produces elemental sulfur, which is recovered as liquid sulfur and water vapor. Traditional Claus systems remove greater than 95% of the sulfur from the process stream. Tail gas processes are used to remove remaining quantities of sulfur compounds to obtain an overall recovery of up to 99.9%.
Various processes are taught for treating tail gas to remove the remaining hydrogen sulfide and sulfur dioxide. The Shell Claus Off-gas Treating process, often referred to as the SCOT process, reacts hydrogen with remaining sulfur dioxide to generate hydrogen sulfide which is in turn absorbed in an amine compound. A typical SCOT process includes a pre-heater 82 for heating the tail gas, a hydrogenation reactor 84, a quench tower 86 to remove water from the tail gas, an amine tower 88 for reaction of the amine solution with the tail gas, a regenerator 96 to strip the hydrogen sulfide for transmission back to the Claus reactor, and an incinerator 92 for burning off treated tail gas. The SCOT process is effective in further reducing sulfur dioxide emissions. However, the SCOT involves substantial capital and operating expense. The hydrogenation reactor 84, required to react hydrogen with hydrogen sulfide and sulfur dioxide, is expensive because of high initial capital cost and operating costs.
U.S. Pat. No. 5,021,232 issued to Hise et al. on Jun. 4, 1991 discloses a process for the cleanup of sulfur-containing constituents in a gaseous stream such as a tail gas from a sulfur recovery unit. A Claus reaction is used to convert sulfur-containing compounds to elemental sulfur in the presence of a stoichiometric excess of hydrogen sulfide. The elemental sulfur is separated from the tail gas and the sulfurous compounds remaining in the tail gas are separated by crystallization for recycle through the Claus process. Carbon dioxide is the crystallization material. Sulfur-containing compounds are at least partially excluded from a solid (frozen) phase of the carbon dioxide for recycling through the Claus process.
U.S. Pat. No. 5,741,469 issued to Bhore et al. on Apr. 21, 1998 discloses a process that may be used to treat Claus plant tail-gas utilizing solid oxides to remove sulfur oxides from gas streams.
U.S. Pat. App. No. US 2003/0082096, invented by Lynn and published on May 1, 2003 discloses treating sulfur dioxide-rich gas by combusting it with a substoichiometric amount of oxygen to produce a combustion gas with water vapor and sulfur vapor. The combustion gas is cooled to form water containing suspended solid sulfur and polythionic acids.
U.S. Pat. No. 6,610,264 issued to Buchanan et al. on Aug. 26, 2003 discloses a process and system for removing sulfur from tail-gas emitted from a Claus sulfur recovery process. The tail-gas is first oxidized so as to convert sulfur therein to sulfur oxides. Oxidized tail-gas is directed into an absorber where a solid absorbent absorbs substantially all the sulfur oxides thereon. After allowing sufficient time for a desired amount of sulfur oxides to be absorbed, absorption is ceased. Next, the solid absorbent containing the absorbed sulfur oxides is contacted with a reducing gas so as to release an off-gas containing hydrogen sulfide and sulfur dioxide. Upon releasing sulfur from the solid absorbent, the solid absorbent is regenerated and redirected into the absorber. Sulfur in the off-gas emitted by regeneratio is concentrated to an extent sufficient for use within a Claus sulfur recovery process for conversion to elemental sulfur.